remove enrollment.sis_user_id since it can't be trusted

refs CNVS-29138

test plan
 - specs should pass

Change-Id: Ie2b703cbc61198dd44a2f9cebe351bc28df1db01
Reviewed-on: https://gerrit.instructure.com/78718
Tested-by: Jenkins
Reviewed-by: Cody Cutrer <cody@instructure.com>
Product-Review: Rob Orton <rob@instructure.com>
QA-Review: Rob Orton <rob@instructure.com>

Author: roor0

app/models/enrollment.rb

@@ -1185,21 +1185,6 @@ def self.cross_shard_invitations?
     false
   end
 
-  # DO NOT TRUST
-  # This is only a convenience method to assist in identifying which enrollment
-  # goes to which user when users have accidentally been merged together
-  # This is the *only* reason the sis_source_id column has not been dropped
-  def sis_user_id
-    return @sis_user_id if @sis_user_id
-    sis_source_id_parts = sis_source_id ? sis_source_id.split(':') : []
-    if sis_source_id_parts.length == 4
-      @sis_user_id = sis_source_id_parts[1]
-    else
-      @sis_user_id = sis_source_id_parts[0]
-    end
-    @sis_user_id
-  end
-
   def total_activity_time
     self.read_attribute(:total_activity_time).to_i
   end

spec/models/enrollment_spec.rb

@@ -1916,14 +1916,6 @@ def enrollment_dates_override_test
     end
   end
 
-  describe "#sis_user_id" do
-    it "should work when sis_source_id is nil" do
-      course_with_student(:active_all => 1)
-      expect(@enrollment.sis_source_id).to be_nil
-      expect(@enrollment.sis_user_id).to be_nil
-    end
-  end
-
   describe "updating cached due dates" do
     before :once do
       course_with_student